This invention relates to an improved Claus catalyst and methods for making such a catalyst. More particularly, it provides an improved Claus catalyst made from activated alumina and sodium oxide; the catalyst possesses increased resistance to sulfate poisoning and higher catalytic activity with respect to compounds such as H.sub.2 S, SO.sub.2, COS and CS.sub.2 than the catalysts of the prior art.
Many industrial fuels contain sulfur compounds which are toxic, corrosive and produce sulfur dioxide when burned. It is necessary, therefore, to remove these sulfurous materials for economic and ecological reasons prior to utilization of the fuel. In the case of crude oil, for example, the oil is typically subjected to hydrodesulfurization (i.e. treatment with hydrogen and a cobalt-molybdenum on alumina catalyst) to produce hydrogen sulfide in conjunction with ammonia, water and diluents. In the case of sour natural gas, hydrogen sulfide and carbon dioxide are usually present in concentrations which can be removed by coventional sweetening processes such as those described in C. D. Swaim, Jr's. article entitled "Gas Sweetening Processes of the 1960's", found in Hydrocarbon Processing, 49(3), 127 (1970). Sweetened sour natural gas by-products and the off-gas from hydrodesulfurization of crude oil are each rich in hydrogen sulfide and may be used, therefore, as feed gases for the well-known Claus process. Basically, in the Claus process, one-third of the total hydrogen sulfide present in the gas to be treated is burned in a furnace with air to produce sulfur dioxide at temperatures between 900.degree. and 1200.degree. C. The remaining two-thirds of the hydrogen sulfide react on the catalyst at temperatures of 200.degree. to 400.degree. C. with the sulfur dioxide so produced to form sulfur and water vapors. A low temperature Claus process is used to condense sulfur on the catalyst at temperatures of about 25.degree. to 200.degree. C. However, at high furnace temperatures, side reactions in which COS and CS.sub.2 are formed also occur. These carbon-sulfur compounds may be removed by catalytic reaction with sulfur dioxide to form carbon dioxide and sulfur and to a lesser degree by catalytic reaction with water vapor to form carbon dioxide and hydrogen sulfide.
Although the amount of COS and CS.sub.2 formed by high temperature side reactions may only amount to a few percent of the sulfurous material present in the emitting furnace gas, increasingly more stringent air regulations make their removal from the gas necessary. Conversion of these carbon-sulfur compounds to sulfur is difficult, however, because of their slow reaction rates. Further, the optimum reaction conditions for each of the carbon-sulfur compounds differ significantly. Removal of these organic sulfur compounds is also complicated by the fact that the presence of sulfurous gas inhibits hydrolysis of the carbon derivatives of sulfur, and, as already indicated, it is believed that hydrolysis is partially responsible for conversion of these carbon-sulfur compounds. Therefore, only the most active of catalysts would be capable of removing these organic sulfur compounds after sulfate formation from sulfur dioxide. Considerable efforts to find such an extraordinarily active catalyst have been exerted in the past few years. Renault et al. (U.S. Pat. No. 3,845,197) describes a process of first reacting the gas stream containing carbon-sulfur compounds such as COS and CS.sub.2 with steam while passing the gas stream over an alumina-containing catalyst at 250.degree. to 400.degree. C. to produce H.sub.2 S. A portion of the H.sub.2 S produced is then oxidized at 300.degree. to 500.degree. C. to produce SO.sub.2 in an amount sufficient to establish a ratio of H.sub.2 S to SO.sub.2 of 1.6 to 3. The SO.sub.2 is then reacted with the remaining portion of H.sub.2 S at 20.degree. to 160.degree. C. to produce elemental sulfur. The catalyst used by Renault et al. to produce the H.sub.2 S is alumina in which one or more metals such as molybdenum, tungsten, iron, nickel or cobalt, may be present as oxides. The catalyst has an alkali metal content lower than 0.1%, a specific surface area of 40 to 500 m.sup.2 /g and a pore volume of 10 to 80 cc/100 g. Oxidation of the H.sub.2 S to SO.sub.2 is then carried out in the presence of a second catalyst (i.e. oxidation catalyst) which may be alumina in which chromium, vanadium, iron or mixtures thereof are present. The oxidized gas stream is then contacted at 20.degree. to 160.degree. C. with an organic solvent which contains a catalyst favoring the reaction between H.sub.2 S and SO.sub.2. The catalyst described as useful in this stage of the treatment is an alkali metal compound. The Renault et al approach divides the gas treatment into three separate stages with a different catalyst for each of these stages. Such a detailed procedure is both expensive and difficult to use in a commercial operation.
Pearson et al. (U.S. Pat. No. 3,725,531) discloses a less complicated process for treating off-gases containing organic sulfur compounds in which the off-gas is contacted with an alumina base catalyst to convert the organic sulfur materials to carbon dioxide and elemental sulfur. The catalysts described as useful in the practice of the Pearson et al. process include an alumina base support in combination with at least one metal selected from strontium, calcium, magnesium, zinc, cadmium, barium and molybdenum. These catalysts, it is claimed, have a high resistance to sulfate poisoning, i.e. the buildup of sulfate on the surface of the catalyst due to oxidation of sulfur dioxide on the active sites of the catalyst employed. Pearson et al. state that suitable alumina base supports for the catalyst include activated bauxite, activated aluminas possessing an essentially chi-rho structure, calcined Bayer hydrate, calcined gel-derived aluminas containing a substantial portion of pseudoboehmite and gamma alumina. It is the promoter (i.e. Ca, Mg, Cd, etc.), however, which acts as an antipoisoning agent to provide increased alumina resistance to sulfate poisoning. However, the amount of antipoisoning agent included in the catalyst, and consequently the effectiveness of the Pearson et al. catalyst, is substantially dependent upon economy of manufacture.
Daumas et al. also disclose improved Claus catalysts in U.S. Pat. Nos. 3,978,004, 4,054,642 and 4,141,962 in which an activated alumina comprises the largest component. In U.S. Pat. No. 3,978,004, the activated alumina is combined with a compound of lanthanum, a lanthanide series metal of atomic number 58 to 71 or a metal of Group IIIB. In U.S. Pat. No. 4,054,642, the alumina is combined with a metal of Group IIIA of the periodic chart. And in U.S. Pat. No. 4,141,962, the alumina is combined with a titanium compound. These promoters (like the Pearson et al promoters) are rather esoteric solutions to the problem of sulfate poisoning of the catalyst. Though a variety of promoters have been tried, there still exists a need for an alumina Claus catalyst which is highly resistant to sulfate poisoning.
In view of the above-discussed problems, it would be particularly advantageous to have available a Claus catalyst which is highly resistant to sulfate poisoning, is relatively cheap to make (i.e. requires no expensive promoter), increases catalytic activity and requires no complication of the standard Claus conversion procedure.